This invention relates to an infinitely variable transmisssion of the type that the transmission ratio is controllable through change of the pitches of the pulleys.
An infinitely variable transmission is already known in which a driving shaft and a driven shaft are connected by means of an endless belt wound around a driving pulley provided on the driving shaft and a driven pulley provided on the driven shaft, the driving pulley and the driven pulley have respective movable pulley elements mounted, respectively, on the driving shaft and the driven shaft for sliding movement thereon by means of a hydraulic fluid so as to vary the pitches of the respective driving shaft and the driven shaft to thereby change the velocity ratio between the driving and driven shafts, i.e. the transmission ratio, and a hydraulic clutch connected to the driven shaft and adapted to become engaged to permit transmission of power from a prime mover such as an internal combustion engine to the driving shaft when the rotational speed of the engine is higher than a predetermined value.
In the conventional infinitely variable transmission, the pitches of the driving and driven pulleys are controlled by means of a hydraulic fluid (pressurized oil) supplied from a hydraulic pressure source, to such values as to achieve velocity ratios between the driving and driven shafts appropriate to operating conditions of the prime mover such as an internal combustion engine. It is a requisite that appropriate pinch forces should be applied to the movable pulley elements upon setting of the pitches of the pulleys, so as not to degrade the power transmission efficiency even at the time of changing the transmission ratio.
To meet this requirement, it has been proposed, e.g. by Japanese Provisional Patent Publication No. 52-98861 to apply a required a pinch force to the movable pulley element of the driven pulley, and at the same time apply to the movable pulley element of the driving pulley a pinch force which is larger than or smaller than the pinch force applied to the movable pulley element of the driven pulley, by a fixed amount at the time of changing the transmission ratio. It has also been proposed, e.g. by U.S. Pat. No. 3,600,961, to create a higher pressure hydraulic fluid having a fixed higher pressure and a lower pressure hydraulic fluid having a lower pressure relative to the fixed higher pressure and variable in response to the transmission ratio assumed or a like parameter, the lower pressure hydraulic fluid being utilized to control a pintch force applied to the movable pulley element of the driving pulley or the driven pulley, wherein the higher pressure hydraulic fluid is applied to the movable pulley element that pushes forward the endless belt, whereas the lower pressure hydraulic fluid is applied to the other movable pulley element that receives the endless belt thus pushed forward. FIG. 6 shows a pinch force characteristic according to the former proposal, and FIG. 7 one according to the latter proposal, respectively.
However, in either of the two proposal, a higher pinch force than an actually required one is obtained in a certan transmission ratio region, resulting in large pressure loss. Furthermore, according to the former proposal the same hydraulic pressure is applied to both the driving pulley and the driven pulley, requiring a large piston area difference commensurate with the transmission ratio changing load and accordingly necessitating making one of the movable pulley elements large in size. On the other hand, according to the latter proposal the use of the higher pressure hydraulic fluid makes it necessary to employ a high pressure-type oil pump as the hydraulic fluid supply source.
Another problem with the aforementioned conventional infinitely variable transmission is as follows: The infinitely variable transmission receives power transmitted from the prime mover or internal combustion engine to be driven thereby when the hydraulic clutch becomes engaged. Therefore, it is necessary to brake the transmission at its output side so as to allow warming-up operation of the engine at a somewhat high rotational speed, which, however, forms an unnecessarily large load on the transmission system. This problem can be solved by designing the transmission to have a neutral position to be assumed during warming-up operation of the engine. However, this solution cannot be applied to the aforementioned infinitely variable transmission, because it is structurally difficult to provide such a neutral position in this type transmission. This is why a mechanical type or gear type transmission with a neutral position is additionally provided in combination with the infinitely variable transmission.
Still another problem with the conventional inifinitely variable transmission is as follows: The movable pulley element of either of the driving pulley and driven pulley is provided with a first hydraulic pressure chamber disposed to have hydraulic pressure therein urge the movable pulley element toward the stationary pulley element, a second hydraulic pressure chamber adjacent to the first hydraulic pressure chamber, and a restriction passage communicating between the first and second hydraulic pressure chambers, whereby part of the hydraulic fluid within the first hydraulic pressure chamber leaks through the restriction passage into the second hydraulic pressure chamber so as to cancel an excessive pressure force in excess of the required initial set pressure force, produced in the first hydraulic pressure chamber due to a centrifugal force caused by rotation of the pulley. The above restriction passage is formed in a stationay piston defining part of the first hydraulic pressure chamber. However, the operation of drilling the restriction passage is rather complicated and difficult to perform, requiring an expensive tool.